This disclosure is related to the field of pulsed neutron well logging apparatus and methods. More specifically, the disclosure is related to using pulsed neutron measurements to evaluate the condition of metal conduit (casing or liner) installed in wellbores drilled through subsurface formations.
Wellbores drilled through subsurface formations for the purpose of producing fluids, e.g., gas and/or oil may have a steel conduit such as a casing (a conduit that extends from the bottom of the wellbore to the surface) or a liner (a conduit that extends from the bottom of the well and is sealingly engaged with a shallower depth conduit installed in the wellbore). The casing or liner is used to protect the mechanical integrity of the wellbore and to provide hydraulic isolation of the various formations penetrated by the wellbore, among other purposes. Because casing or liner is typically made from various steel alloys, it is subject to corrosion.
It is important, for example, in older wells or in wells where corrosive fluids are being produced to determine not only the properties of the formation or the borehole fluid, but also of the casing or liner itself. Various devices are known in the art for the inspection of the thickness and surface quality (e.g. pitting, roughness or holes due to corrosion). Examples of such devices include: (i) ultrasonic logging tools (generally scan a transmitter/receiver pair and look for ultrasonic reflection); (ii) electromagnetic logging tools (measure total amount of metal present via eddy currents; (iii) multiple contact arm (“finger”) caliper tools (inspect the internal surface of the casing by tracing it with a multitude of measurement fingers; and (iv) casing collar locator tools (very qualitative, but spikes in areas without casing collars can indicate casing issues).
The instruments described above may require that a wellbore tubing (a conduit having smaller diameter than the liner or casing used to increase fluid velocity from the wellbore) has to be removed from the wellbore a priori, which may be expensive and inconvenient for routine casing or liner inspection. The foregoing instruments provide no other information than that about the condition of the casing or liner (although ultrasonic inspection may provide indications of quality of the cement used to retain the casing or liner, and the caliper tool may provide indication of scale buildup inside the casing).
Unless there is a specific wellbore maintenance and monitoring plan, one usually only finds out about a problem after it occurs, because the above described casing/liner evaluation devices are typically used to characterize a problem that has already manifested itself as a problem with the wellbore (generally because of the above described inconvenience and expense of removing wellbore tubing for inspection purposes using the above described instruments).